1. Field of the Invention
This invention relates to a method of making pillared hydrotalcites and associated products, and more particularly it relates to making pillared hydrotalcites by calcining a hydrotalcite compound and combining the calcined compound with a salt solution of an anion and the resultant products.
2. Background Information
Hydrotalcites are layered double hydroxide compounds of the type EQU A.sub.w B.sub.x (OH).sub.y [C].sub.z .multidot.nH.sub.2 O
where A represents a divalent metal cation, B represents a trivalent metal cation and C represents a mono- to tetravalent anion. The structure of these compounds consists of brucite-like layers of the metal hydroxide between which the anions are sandwiched. One example of a hydrotalcite compound is EQU Mg.sub.6 Al.sub.2 (OH).sub.16 CO.sub.3 .multidot.4H.sub.2 O.
Some hydrotalcites are found naturally but may also be synthesized for commercial use. One method used for synthesis is by combining activated magnesia and aluminate liquors. See U.S. Pat. No 4,904,457, the disclosure of which is fully incorporated by reference herein
The "pillaring" of certain metal hydroxide compounds is known. Pillaring has been used extensively with respect to cation replacement in smectite clays. Hydrotalcite pillaring involves modifying the brucite-like molecular layers of the hydrotalcite with an organic, inorganic or mixed organic/inorganic anion. Pillared hydrotalcites are useful as adsorbents, catalysts and molecular sieves.
U.S. Pat. No. 4,774,212 describes several pillared hydrotalcite structure compositions wherein the pillared anion can be (i) an organic anion selected from the group consisting of lauryl sulfate, p-toluenesulfonate and 1,5-naphthalene disulfonate; (ii) an inorganic anionic polyoxometalate selected from the group consisting of polyoxometalates of vanadium, tungsten and molybdenum; or (iii) a mixed organic/inorganic anion.
U.S. Pat. No. 4,774,212 also discloses a process for making pillared compositions comprising contacting a hydrotalcite structure composition having a large organic anion selected from the group consisting of p-toluenesulfonate, terephthalate, 2,5-dihydroxy-1,4-benzene-disulfonate and 1,5-naphthalenedisulfonate with a solution containing polyoxometalates of vanadium, tungsten and molybdenum at a pH of between about 3 and 6 and temperature between about 20.degree. C. and 100.degree. C. In this process it is said that at least some of the large organic anions are substituted by the anion of the polyoxometalate. This patent further discloses heating a pillared composition of the hydrotalcite structure which contains both large organic and inorganic anions until substantially all the large organic anion has been removed therefrom.
The pillaring process disclosed in U.S. Pat. No. 4,774,212 involves replacing the carbonate anion from the hydrotalcite first with an organic anion and then with an inorganic anion. As will be appreciated, this process involves multiple steps using different organic and inorganic anions.
There remains a need for a more efficient and commercially viable means for producing pillared hydrotalcite which involves less process steps while maintaining the usefulness and integrity of the final product.